When an existent scanner scans a piece of paper, the scanning area is predetermined by the position and size of an adjustable tray. Therefore, the paper must be put in the correct position so it can be scanned properly. If the paper is not put properly, the boundary of the paper outsteps the region of the adjustable tray, and a portion of the gotten image is cut off, even though the size of the paper is not larger than the adjustable tray. On the other hand, if the size of the paper is much smaller, it is unwise to convert so many electronic signals into an image picture and then cut the unwanted margin which, however, needs to occupy some memory space. Accordingly, a sensor is conventionally added to detect the position of the paper, but the prior art has the following disadvantages.
1. Assembling parts into a scanner introduces errors in the relative positions of the elements. Although the piece of paper is put in the stated position, the image still shifts. So we must adjust the position of the piece of paper to be scanned again.
2. After detecting the position of the piece of paper, the position of the charge coupled device will be adjusted and unavoidable errors between elements generate. The long adjusting time is also a trouble.